1. Field of the Invention
The present invention relates to a water-insoluble, amorphous, alkali metal aluminosilicophosphate having a large surface area, which can sequester divalent metal ions with high efficiency and can effectively absorb therein gases or solute molecules.
It is a primary object of the present invention to overcome the defect of crystalline aluminosilicates, i.e., zeolites, that are widely used as adsorbents or metal ion sequestering agents in various industries, namely, their lack of a sufficient magnesium ion-sequestering capacity.
More specifically, because zeolites generally have a large surface area, they possess an excellent capacity for adsorbing gas molecules and solute molecules in steam and water and an excellent capacity for sequestering some divalent metal ions, such as calcium, in water, but they are fatally defective for many purposes because the magnesium ion-sequestering capacity thereof is extremely low. The present invention overcomes this defect and provides a metal sequestering agent having a high magnesium ion-sequestering capacity, while maintaining other properties similar to those of conventional zeolites.
2. Description of the Prior Art
Water-soluble, metal ion-sequestering agents exemplified by various phosphates (especially condensed phosphates such as pyrophosphates and tripolyphosphates), aminotricarboxylic acid salts such as ethylenediamine tetraacetates (EDTA) and nitrilotriacetates (NTA), hydroxycarboxylic acid salts such as citric acid salts and gluconic acid salts, and various polymeric electrolytes such as polyacrylic acid salts and alkali-hydrolyzed maleic anhydride-vinyl compound copolymers, are water-soluble and they cannot adsorb solute molecules after sequestering metal ions. In contrast, water-insoluble metal sequestering agents can adsorb solute molecules after sequestering metal ions. Furthermore, water-soluble sequestering agents are defective in the feature that when they are incorporated in a solid (powdery) state into powdery substrates, when the water content of the mixture is increased, the mixture becomes sticky and viscous. In contrast, water-insoluble metal sequestering agents do not have that disadvantage and they possess the capacity of adsorbing gases and solute molecules. Because of these excellent properties of water-insoluble metal sequestering agents, there have heretofore been proposed and used the following water-insoluble metal sequestering agents:
(i) Cation exchange resins (for example, Diaion manufactured by Mitsubishi Kasei and Amberlite IR series manufactured by Rohm & Haas) PA1 (ii) Fibrous polymers PA1 (iii) Clays (for example, montmorillonite Al.sub.2 [Si.sub.4 O.sub.10 (OH).sub.2 ].nH.sub.2 O and beidellite Al.sub.2 [(OH).sub.2 AlSi.sub.3 O.sub.9 OH].4H.sub.2 O) PA1 (iv) Crystalline aluminosilicates (for example, Molecular Sieves 3A, 4A and 13X). PA1 0.7.ltoreq.x.sub.1 .ltoreq.1.5, and PA1 0.8.ltoreq.y.sub.1 .ltoreq.6, PA1 0.5.ltoreq.x.sub.2 .ltoreq.1.1, PA1 0.8.ltoreq.y.sub.2 .ltoreq.2.5, and PA1 0.01.ltoreq.z.sub.2 .ltoreq.0.80,
Among these water-insoluble metal sequestering agents, crystalline aluminosilicates (zeolites) are cheap and available in large quantities and they can be used on an industrial scale. It is known that among the class of crystalline aluminosilicates, a synthetic crystalline zeolite having the following formula (A) is most preferred: EQU [x.sub.1 (M.sub.2 O or MO).Al.sub.2 O.sub.3.y.sub.1 (SiO.sub.2).z.sub.1 (H.sub.2 O)] (crystalline) (A)
wherein M is an alkali metal or alkaline earth metal, and x.sub.1 and y.sub.1 are numbers representing the mole numbers of the respective components, and x.sub.1 and y.sub.1 generally satisfy the following relations:
and z.sub.1 is an optional positive number.
Specific examples of synthetic crystalline zeolites of this type include Molecular Sieve 3A [(Na.sub.2 O,K.sub.2 O).Al.sub.2 O.sub.3.2(SiO.sub.2).4.5(H.sub.2 O)], Molecular Sieve 4A [(Na.sub.2 O).Al.sub.2 O.sub.3.2(SiO.sub.2).4.5(H.sub.2 O)] and Molecular Sieve 13X [(Na.sub.2 O).Al.sub.2 O.sub.3.2.46(SiO.sub.2).6.42(H.sub.2 O)].
Crystalline aluminosilicophosphates represented by the following general formula (B): EQU [x.sub.2 (M.sub.2 O).Al.sub.2 O.sub.3.y.sub.2 (SiO.sub.2).z.sub.2 (P.sub.2 O.sub.5).w.sub.2 (H.sub.2 O)] (crystalline) (B)
wherein M is sodium or potassium, and x.sub.2, y.sub.2, z.sub.2 and w.sub.2 are numbers representing the mole numbers of the respective components, wherein x.sub.2, y.sub.2 and z.sub.2 satisfy the following relations:
and w.sub.2 is an optional positive number including zero, are known as adsorbents. Although these crystalline aluminosilicophosphates have not been used as metal sequestering agents, our research has revealed that they have a sequestering activity for divalent metal ions.
A process for the synthesis of crystalline aluminosilicophosphates represented by the general formula (B) is disclosed in Japanese Patent Publication No. 29579/70.
Because crystalline aluminosilicates represented by the above general formula (A) and crystalline aluminosilicophosphates represented by the above general formula (B) have a large surface area owing to crystallographic voids (fine pores of molecular size), they have a high adsorbing capacity and a high metal ion-sequestering capacity, but they have the defect that the magnesium ion-sequestering capacity thereof is low.